Preheat the oven to 325 degrees F (165 degrees C). Grease a 9x13-inch casserole dish.

Fill a large pot with lightly salted water and bring to a rolling boil. Stir in mostaccioli and return to a boil. Cook mostaccioli uncovered, stirring occasionally, until tender yet firm to the bite, 8 to 10 minutes. Drain well.

Meanwhile, cook ground beef and onion in a skillet over medium heat until browned.

Combine spaghetti sauce, mozzarella cheese, mushrooms, mostaccioli, and ground beef mixture in a large bowl; transfer to the prepared casserole dish.

Bake in the preheated oven until very hot, about 20 minutes.